1. Field
An electrolyte for a rechargeable lithium battery and a rechargeable lithium battery including the same are disclosed.
2. Description of the Related Art
Charges are moved by electrons in an external electrical conductor during charge and discharge of a rechargeable lithium battery but by lithium ions between electrically separated positive and negative electrodes. An electrolyte for a rechargeable lithium battery may be defined as medium through which ion movement is performed.
The electrolyte may include a functional additive, and the functional additive is included in a small amount of several %'s based on the total amount of the electrolyte and thus, may not largely change properties of the electrolyte, and include various compounds. The functional additive may be various but classified into two types based on the operation mechanism.
The first type of the functional additive has an influence on interface characteristics between electrode and electrolyte and plays a role of forming a protection layer through an electrochemical or physical/chemical reaction or adsorption on the surface of a positive or negative electrode and suppressing battery deterioration or improving safety and the like.
The second type of the functional additive improves characteristics of an electrolyte itself and directly works on a solvent or a lithium salt and plays a role of increasing stability or ion conductivity or removing impurities impeding stability of the electrolyte (moisture, HF, and other byproducts). In general, main characteristics (energy/power, cycle-life, safety, and the like) of a rechargeable lithium battery is primarily determined by positive and negative electrodes. However, these characteristics may be realized by using an appropriate electrolyte and maximized by selecting an optimal electrolyte composition.
A conventional carbonate-based electrolyte for a rechargeable lithium battery is oxidized at a much higher voltage than operation potential of most positive electrode materials. Accordingly, the electrolyte is hardly oxidized under a normal charge and discharge condition which is no extreme condition such as exposure of a battery to a high temperature, its overcharge, and the like. However, recent research results show that decomposition byproducts of the electrolyte are present on the surface of the positive electrode.
These electrolyte decomposition byproducts may be produced by a chemical reaction but mostly by an electrochemical oxidation reaction of the electrolyte. The oxidization of the electrolyte on the positive electrode during normal charge and discharge is regarded as a local voltage increase or temperature increase of the positive electrode. In particular, sharply-increased impedance of a positive electrode rather than a negative electrode generally deteriorates cycle-life as the repeated charge and discharge cycles, and herein, the impedance is known to be mainly increased when byproducts due to oxidation of the electrolyte are piled up on the surface of the positive electrode. The surface change rather than bulk characteristics of the positive electrode is known to be mainly deteriorate battery performance, and accordingly, many attempts have been made to control surface reactivity by modifying the surface of the positive electrode.
Development of an electrolyte having low chemical or electrochemical reactivity with the positive electrode is needed. However, some degrees of reactivity with the positive electrode is preferable in terms of safety. The large amount of byproducts on the surface of the positive electrode may deteriorate battery performance, but appropriate amount of the byproducts may contribute to safety of the positive electrode.